Return a success (zero) exit code from self:update if there is no update

Closes #1135

Author: pjcdawkins

src/Command/CommandBase.php

@@ -336,6 +336,9 @@ protected function checkUpdates()
 
         try {
             $newVersion = $cliUpdater->update(null, $currentVersion);
+            if ($newVersion === '') {
+                return;
+            }
         } catch (\RuntimeException $e) {
             if (strpos($e->getMessage(), 'Failed to download') !== false) {
                 $this->stdErr->writeln('<error>' . $e->getMessage() . '</error>');

src/Command/Self/SelfUpdateCommand.php

@@ -34,6 +34,9 @@ protected function execute(InputInterface $input, OutputInterface $output)
         $cliUpdater->setTimeout($input->getOption('timeout'));
 
         $result = $cliUpdater->update($manifestUrl, $currentVersion);
+        if ($result === '') {
+            return 0;
+        }
         if ($result === false) {
             return 1;
         }

src/Service/SelfUpdater.php

@@ -80,7 +80,7 @@ public function setAllowMajor($allowMajor = true)
      * @param string|null $currentVersion
      *
      * @return false|string
-     *   The new version number, or false if there was no update.
+     *   The new version number, or an empty string if there was no update, or false on error.
      */
     public function update($manifestUrl = null, $currentVersion = null)
     {
@@ -124,7 +124,7 @@ public function update($manifestUrl = null, $currentVersion = null)
 
         if (!$updater->hasUpdate()) {
             $this->stdErr->writeln('No updates found');
-            return false;
+            return '';
         }
 
         $newVersionString = $updater->getNewVersion();